Mitochondrial substrate availability and its role in lipid-induced insulin resistance and proinflammatory signaling in skeletal muscle

Investigating the role of mitochondrial DNA D-loop variants, haplotypes, and copy number in polycystic ovary syndrome: implications for clinical phenotypes in the Chinese population

Background

The presence of genetic variations in mitochondrial DNA (mtDNA) has been associated with a diverse array of diseases. The objective of this study was to examine the correlations between mtDNA D-loop, its haplotypes, and polycystic ovary syndrome (PCOS) in the Chinese population, and the associations between mtDNA D-loop and symptoms of PCOS. The study also sought to determine whether the mtDNA copy number in Chinese patients with PCOS differed from that of individuals in the control group.

Methods

Infertile individuals who only had tubal or male factor treatment were the focus of research by The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO). mtDNA haplotypes were categorized using polymorphic D-loop sites. mtDNA D-loop, PCOS features, and mtDNA haplotypes were analyzed using R software to determine the strength of the association between the three. There are certain DNA haplotypes linked to PCOS. Microdroplet digital polymerase chain reaction (PCR) was used to determine the mtDNA copy number in a convenience sample of 168 PCOS patients and 83 controls.

Results

Among the research group, the majority of D-loop mutations were infrequent (frequency< 1%), with only 45 variants displaying a minimum allele frequency (MAF) of 5% or higher. No association was found between polymorphism loci in PCOS patients and body mass index (BMI). Noteworthy, C194T, 1A200G, 523delAC, and C16234T showed positive correlations with elevated LH/FSH levels. Additionally, specific polymorphic loci G207A, 16036GGins, and 16049Gins within the D-loop region of mtDNA potentially exerted a protective role in PCOS development. Conversely, no statistical significance was observed in the expression levels of C16291T and T489C. Chinese women with mtDNA haplotype A15 exhibited a decreased risk of developing PCOS. Moreover, a significant difference in mtDNA copy number was detected, with controls averaging 25.87 (21.84, 34.81), while PCOS patients had a mean of 129.91 (99.38, 168.63).

Conclusion

Certain mtDNA D-loop mutations and haplotypes appear to confer protection against PCOS in Chinese women. In addition, elevated mtDNA copy number may serve as an indicator during early stages of PCOS.

Mitophagy: A potential therapeutic target for insulin resistance

Glucose and lipid metabolism disorders caused by insulin resistance (IR) can lead to metabolic disorders such as diabetes, obesity, and the metabolic syndrome. Early and targeted intervention of IR is beneficial for the treatment of various metabolic disorders. Although significant progress has been made in the development of IR drug therapies, the state of the condition has not improved significantly. There is a critical need to identify novel therapeutic targets. Mitophagy is a type of selective autophagy quality control system that is activated to clear damaged and dysfunctional mitochondria. Mitophagy is highly regulated by various signaling pathways, such as the AMPK/mTOR pathway which is involved in the initiation of mitophagy, and the PINK1/Parkin, BNIP3/Nix, and FUNDC1 pathways, which are involved in mitophagosome formation. Mitophagy is involved in numerous human diseases such as neurological disorders, cardiovascular diseases, cancer, and aging. However, recently, there has been an increasing interest in the role of mitophagy in metabolic disorders. There is emerging evidence that normal mitophagy can improve IR. Unfortunately, few studies have investigated the relationship between mitophagy and IR. Therefore, we set out to review the role of mitophagy in IR and explore whether mitophagy may be a potential new target for IR therapy. We hope that this effort serves to stimulate further research in this area.

Molecular Mechanism of Lipotoxicity as an Interesting Aspect in the Development of Pathological States-Current View of Knowledge

Free fatty acids (FFAs) play numerous vital roles in the organism, such as contribution to energy generation and reserve, serving as an essential component of the cell membrane, or as ligands for nuclear receptors. However, the disturbance in fatty acid homeostasis, such as inefficient metabolism or intensified release from the site of storage, may result in increased serum FFA levels and eventually result in ectopic fat deposition, which is unfavorable for the organism. The cells are adjusted for the accumulation of FFA to a limited extent and so prolonged exposure to elevated FFA levels results in deleterious effects referred to as lipotoxicity. Lipotoxicity contributes to the development of diseases such as insulin resistance, diabetes, cardiovascular diseases, metabolic syndrome, and inflammation. The nonobvious organs recognized as the main lipotoxic goal of action are the pancreas, liver, skeletal muscles, cardiac muscle, and kidneys. However, lipotoxic effects to a significant extent are not organ-specific but affect fundamental cellular processes occurring in most cells. Therefore, the wider perception of cellular lipotoxic mechanisms and their interrelation may be beneficial for a better understanding of various diseases’ pathogenesis and seeking new pharmacological treatment approaches.

Cannabinoid Receptors Overexpression in a Rat Model of Irritable Bowel Syndrome (IBS) after Treatment with a Ketogenic Diet

The administration of a ketogenic diet (KD) has been considered therapeutic in subjects with irritable bowel syndrome (IBS). This study aimed to investigate the molecular mechanisms by which a low-carbohydrate diet, such as KD, can improve gastrointestinal symptoms and functions in an animal model of IBS by evaluating possible changes in intestinal tissue expression of endocannabinoid receptors. In rats fed a KD, we detected a significant restoration of cell damage to the intestinal crypt base, a histological feature of IBS condition, and upregulation of CB1 and CB2 receptors. The diet also affected glucose metabolism and intestinal membrane permeability, with an overexpression of the glucose transporter GLUT1 and tight junction proteins in treated rats. The present data suggest that CB receptors represent one of the molecular pathways through which the KD works and support possible cannabinoid-mediated protection at the intestinal level in the IBS rats after dietary treatment.

WY14643 improves left ventricular myocardial mitochondrial and systolic functions in obese rats under chronic persistent hypoxia via the PPARα pathway

AIMS

Peroxisome proliferator-activated receptor (PPAR) α, a key regulator of lipid metabolism, plays a role in maintaining the homeostasis of myocardial energy metabolism. Both hypoxia and obesity inhibit the expression of PPARα in the myocardium. In this study, we verified the inhibitory effects of hypoxia and obesity on PPARα and examined whether WY14643 (4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid), an agonist of PPARα, ameliorates myocardial mitochondrial dysfunction and protects cardiac function in obese rats under chronic persistent hypoxia.

MAIN METHODS

Sprague-Dawley rats were randomly divided into six groups: a control group (normal chow diet, normal oxygen), a high-fat diet (HFD) group (normal oxygen), a chronic persistent hypoxia normal chow diet group, a chronic persistent hypoxia HFD group, a chronic persistent hypoxia HFD group with WY14643 treatment, and a chronic persistent hypoxia HFD group with vehicle treatment.

KEY FINDINGS

Hypoxia and obesity increased myocardial lipid accumulation, mitochondrial dysfunction, and left ventricular systolic dysfunction. Myocardial lipid metabolism-related genes, including those encoding PPARα, PPARγ coactivator 1α (PGC1α), and carnitine palmitoyl transferase 1α (CPT1α), were downregulated, while acetyl-CoA carboxylase 2 (ACC2) was upregulated under a combination of hypoxia and obesity. WY14643 upregulated PPARα, PGC1α, and CPT1α, and downregulated ACC2. WY14643 alleviated hypoxia- and obesity-induced myocardial lipid accumulation and improved mitochondrial and left ventricular systolic functions.

SIGNIFICANCE

WY14643 improved myocardial mitochondrial and left ventricular systolic functions in obese rats under chronic persistent hypoxia. Thus, WY14643 possibly exerts its effects by regulating the PPARα pathway and shows potential as a therapeutic target for cardiovascular diseases associated with obesity and hypoxia.

Endocannabinoid Receptor-1 and Sympathetic Nervous System Mediate the Beneficial Metabolic Effects of Gastric Bypass

The exact mechanisms underlying the metabolic effects of bariatric surgery remain unclear. Here, we demonstrate, using a combination of direct and indirect calorimetry, an increase in total resting metabolic rate (RMR) and specifically anaerobic RMR after Roux-en-Y gastric bypass (RYGB), but not sleeve gastrectomy (SG). We also show an RYGB-specific increase in splanchnic sympathetic nerve activity and "browning" of visceral mesenteric fat. Consequently, selective splanchnic denervation abolishes all beneficial metabolic outcomes of gastric bypass that involve changes in the endocannabinoid signaling within the small intestine. Furthermore, we demonstrate that administration of rimonabant, an endocannabinoid receptor-1 (CB1) inverse agonist, to obese mice mimics RYGB-specific effects on energy balance and splanchnic nerve activity. On the other hand, arachidonoylethanolamide (AEA), a CB1 agonist, attenuates the weight loss and metabolic signature of this procedure. These findings identify CB1 as a key player in energy regulation post-RYGB via a pathway involving the sympathetic nervous system.

Proinflammatory NFkB signalling promotes mitochondrial dysfunction in skeletal muscle in response to cellular fuel overloading

Sustained nutrient (fuel) excess, as occurs during obesity and diabetes, has been linked to increased inflammation, impaired mitochondrial homeostasis, lipotoxicity, and insulin resistance in skeletal muscle. Precisely how mitochondrial dysfunction is initiated and whether it contributes to insulin resistance in this tissue remains a poorly resolved issue. Herein, we examine the contribution that an increase in proinflammatory NFkB signalling makes towards regulation of mitochondrial bioenergetics, morphology, and dynamics and its impact upon insulin action in skeletal muscle cells subject to chronic fuel (glucose and palmitate) overloading. We show sustained nutrient excess of L6 myotubes promotes activation of the IKKβ-NFkB pathway (as judged by a six-fold increase in IL-6 mRNA expression; an NFkB target gene) and that this was associated with a marked reduction in mitochondrial respiratory capacity (>50%), a three-fold increase in mitochondrial fragmentation and 2.5-fold increase in mitophagy. Under these circumstances, we also noted a reduction in the mRNA and protein abundance of PGC1α and that of key mitochondrial components (SDHA, ANT-1, UCP3, and MFN2) as well as an increase in cellular ROS and impaired insulin action in myotubes. Strikingly, pharmacological or genetic repression of NFkB activity ameliorated disturbances in mitochondrial respiratory function/morphology, attenuated loss of SDHA, ANT-1, UCP3, and MFN2 and mitigated the increase in ROS and the associated reduction in myotube insulin sensitivity. Our findings indicate that sustained oversupply of metabolic fuel to skeletal muscle cells induces heightened NFkB signalling and that this serves as a critical driver for disturbances in mitochondrial function and morphology, redox status, and insulin signalling.

Omentin-1 attenuates lipopolysaccharide (LPS)-induced U937 macrophages activation by inhibiting the TLR4/MyD88/NF-κB signaling

Macrophages play a pivotal role in the defense response against harmful pathogens and stimuli by releasing various pro-inflammatory mediators. However, overproduction of pro-inflammatory mediators will do harm to the organism and cause inflammation-associated diseases. Omentin-1, which is a newly discovered adipokine, is specifically expressed in omental adipose tissue. Recent studies have found correlations between omentin-1 and insulin resistance, diabetes, obesity, inflammation, atherosclerosis, bone metabolism, and tumor cell proliferation. Some studies have shown that the association between omentin-1, insulin resistance, and inflammation might suggest that omentin-1 plays an important role in chronic inflammatory diseases. In this study, we found that omentin-1 inhibited LPS-induced expression of inflammatory mediators and pro-inflammatory cytokines in macrophages. Furthermore, omentin-1 inhibited activation of the NF-κB pathway by suppressing both nuclear p65 accumulation and transfected NFκB promoter activity. Importantly, omentin-1 increased nuclear translocation of Nrf2. Our findings demonstrate that omentin-1 exerts anti-inflammatory effects on LPS-induced macrophages and has potential implication in the treatment of inflammation-associated diseases.

The BACE1 product sAPPβ induces ER stress and inflammation and impairs insulin signaling

OBJECTIVE

β-secretase/β-site amyloid precursor protein (APP)-cleaving enzyme 1 (BACE1) is a key enzyme involved in Alzheimer's disease that has recently been implicated in insulin-independent glucose uptake in myotubes. However, it is presently unknown whether BACE1 and the product of its activity, soluble APPβ (sAPPβ), contribute to lipid-induced inflammation and insulin resistance in skeletal muscle cells.

MATERIALS/METHODS

Studies were conducted in mouse C2C12 myotubes, skeletal muscle from Bace1^-/-mice and mice treated with sAPPβ and adipose tissue and plasma from obese and type 2 diabetic patients.

RESULTS

We show that BACE1 inhibition or knockdown attenuates palmitate-induced endoplasmic reticulum (ER) stress, inflammation, and insulin resistance and prevents the reduction in Peroxisome Proliferator-Activated Receptor γ Co-activator 1α (PGC-1α) and fatty acid oxidation caused by palmitate in myotubes. The effects of palmitate on ER stress, inflammation, insulin resistance, PGC-1α down-regulation, and fatty acid oxidation were mimicked by soluble APPβ in vitro. BACE1 expression was increased in subcutaneous adipose tissue of obese and type 2 diabetic patients and this was accompanied by a decrease in PGC-1α mRNA levels and by an increase in sAPPβ plasma levels of obese type 2 diabetic patients compared to obese non-diabetic subjects. Acute sAPPβ administration to mice reduced PGC-1α levels and increased inflammation in skeletal muscle and decreased insulin sensitivity.

CONCLUSIONS

Collectively, these findings indicate that the BACE1 product sAPPβ is a key determinant in ER stress, inflammation and insulin resistance in skeletal muscle and gluconeogenesis in liver.

H55N polymorphism is associated with low citrate synthase activity which regulates lipid metabolism in mouse muscle cells

The H55N polymorphism in the Cs gene of A/J mice has been linked to low activity of the enzyme in skeletal muscles. The aim of the study was to test this hypothesis and examine effects of low citrate synthase (CS) activity on palmitate metabolism in muscle cells. Results of the study showed that carriers of the wild type (WT) Cs (C57BL/6J and Balb/cByJ mouse strains) had higher CS activity (p < 0.01) than carriers of the A/J variant (B6.A-(rs3676616-D10Utsw1)/KjnB6 and A/J mouse strains) in the heart, liver and gastrocnemius muscle. Furthermore, the recombinant CS protein of WT showed higher CS activity than the A/J variant. In C2C12 muscle cells the shRNA mediated 47% knockdown of CS activity reduced the rate of fatty acid oxidation compared to the control cells. In summary, our results are consistent with the hypothesis that H55N substitution causes a reduction in CS activity. Furthermore, low CS activity interferes with metabolic flexibility of muscle cells.

Long-term treatment with nicotinamide induces glucose intolerance and skeletal muscle lipotoxicity in normal chow-fed mice: compared to diet-induced obesity

Nicotinamide (NAM), or vitamin B3, is an essential coenzyme for ATP synthesis and an inhibitor of sirtuin 1. Recently, conflicting results were reported regarding the treatment of NAM in type 2 diabetes and obesity. The aim of this study was to determine whether and how long-term treatment with NAM at lower dose would affect insulin sensitivity in mice fed chow diet. We treated mice with NAM (100 mg/kg/day) and normal chow for 8 weeks. Strikingly, NAM induced glucose intolerance and skeletal muscle lipid accumulation in nonobese mice. NAM impaired mitochondrial respiration capacity and energy production in skeletal muscle, in combination with increased expression of the mediators for mitophagy (p62, PINK1, PARK2 and NIX) and autophagy (FOXO3, Bnip3, CTSL, Beclin1 and LC-3b). Next, we treated mice with high-fat diet (HFD) and resveratrol (RSV; 100 mg/kg/day) for 8 weeks. RSV protected against HFD-induced insulin resistance and obesity. HFD increased skeletal muscle lipid content as well as NAM, but this increase was attenuated by RSV. In contrast to NAM, HFD enhanced fatty acid oxidative capacity. Muscle transcript levels of genes for mitophagy and autophagy were largely suppressed by HFD, whereas RSV did not rescue these effects. These differences suggest that skeletal muscle autophagy may represent adaptive response to NAM-induced lipotoxicity, whereas reduced autophagy in skeletal muscle may promote HFD-induced lipotoxicity. Our results demonstrate that chronic NAM supplementation in healthy individuals, although at lower dose than previously reported, is still detrimental to glucose homeostasis and skeletal muscle lipid metabolism.

Mitochondrial involvement in skeletal muscle insulin resistance: A case of imbalanced bioenergetics

Skeletal muscle insulin resistance in obesity associates with mitochondrial dysfunction, but the causality of this association is controversial. This review evaluates mitochondrial models of nutrient-induced muscle insulin resistance. It transpires that all models predict that insulin resistance arises as a result of imbalanced cellular bioenergetics. The nature and precise origin of the proposed insulin-numbing molecules differ between models but all species only accumulate when metabolic fuel supply outweighs energy demand. This observation suggests that mitochondrial deficiency in muscle insulin resistance is not merely owing to intrinsic functional defects, but could instead be an adaptation to nutrient-induced changes in energy expenditure. Such adaptive effects are likely because muscle ATP supply is fully driven by energy demand. This market-economic control of myocellular bioenergetics offers a mechanism by which insulin-signalling deficiency can cause apparent mitochondrial dysfunction, as insulin resistance lowers skeletal muscle anabolism and thus dampens ATP demand and, consequently, oxidative ATP synthesis.

Cardiac metabolic alterations in hypertensive obese pigs

Obesity and hypertension are major risk factors for cardiovascular diseases, and their growing coexistence accounts for an increase in adverse cardiac events, but the mechanisms are yet to be determined. We hypothesized that obesity exacerbates mitochondrial dysregulation imposed by hypertension and augments left ventricular dysfunction. Obesity-prone Ossabaw pigs were randomized to lean (standard diet) and obese (high-fat diet), without (Lean-sham and Obese-sham) or with renovascular hypertension (Lean-hypertension and Obese-hypertension), induced after 12 weeks of diet (n=7 each). Cardiac function, myocardial perfusion and oxygenation, and microvascular remodeling were assessed 4 weeks later. Mitochondrial biogenesis signals and structural proteins, respiratory chain complex activities, and mitochondrial self-degradation were examined, as was fibrosis. Obesity alone exerted no apparent effect on mitochondrial dynamics, but aggravated in hypertensive hearts the reduction of mitochondrial proteins, deoxyribonucleic acid content, and respiratory chain complex IV subunits activity, and amplified mitochondrial self-degradation. Synergistic interaction of obesity with hypertension also exacerbated myocardial fibrosis and left ventricular diastolic dysfunction. Mitochondrial content, respiratory chain complex IV subunits activity, and mitophagy were correlated with myocardial fibrosis. These findings suggest that obesity aggravates in renovascular hypertension cardiac mitochondrial aberrations. Mitochondrial function may regulate the progression of cardiac injury and functional deterioration in hypertension concomitant with obesity.

NEU3 sialidase as a marker of insulin sensitivity: Regulation by fatty acids

The plasma membrane-associated enzyme NEU3 sialidase functions to cleave sialic acid residues from the ganglioside GM3 thereby promoting its degradation, and has been implicated in the modulation of insulin action. Herein, we report for the first time that impaired insulin sensitivity in skeletal muscle and liver of obese Zucker fatty rats and aged C57BL/6 mice coincides with reduced NEU3 protein abundance. In addition, high fat feeding was found to significantly reduce NEU3 protein in white adipose tissue of rats. Notably, we also demonstrate the ability of the fatty acids palmitate and oleate to repress and induce NEU3 protein in L6 myotubes, concomitant with their insulin desensitising and enhancing effects, respectively. Moreover, we show that the palmitate-driven loss in NEU3 protein is mediated, at least in part, by intracellular ceramide synthesis but does not involve the proteasomal pathway. Strikingly, we further reveal that protein kinase B (PKB/Akt) acts as a key positive modulator of NEU3 protein abundance. Together, our findings implicate NEU3 as a potential biomarker of insulin sensitivity, and provide novel mechanistic insight into the regulation of NEU3 expression.

A multicomponent nutrient bar promotes weight loss and improves dyslipidemia and insulin resistance in the overweight/obese: chronic inflammation blunts these improvements

This study determined if twice-daily consumption of a nutrient-dense bar intended to fill gaps in Western diets, without other dietary/lifestyle requirements, favorably shifted metabolic/anthropometric indicators of dysregulation in a healthy direction. Three 8-wk clinical trials in 43 healthy lean and overweight/obese (OW/OB) adults, who served as their own controls, were pooled for analysis. In less inflamed OW/OB [high-sensitivity C-reactive protein (hsCRP) <1.5], statistically significant decreases occurred in weight (-1.1 ± 0.5 kg), waist circumference (-3.1 ± 1.4 cm), diastolic blood pressure (-4.1 ± 1.6 mmHg), heart rate [HR; -4.0 ± 1.7 beats per minute (bpm)], triglycerides (-72 ± 38.2 mg/dl), insulin resistance (homeostatic model of insulin resistance) (-0.72 ± 0.3), and insulin (-2.8 ± 1.3 mU/L); an increase in HDL-2b (+303 ± 116 nM) and realignment of LDL lipid subfractions toward a less atherogenic profile [decreased small LDL IIIb (-44 ± 23.5 nM), LDL IIIa (-99 ± 43.7 nM), and increased large LDL I (+66 ± 28.0 nM)]. In the more inflamed OW/OB (hsCRP >1.5), inflammation was reduced at 2 wk (-0.66 mg/L), and HR at 8 wk (-3.4 ± 1.3 bpm). The large HDL subfraction (10.5-14.5 nm) increased at 8 wk (+346 ± 126 nM). Metabolic improvements were also observed in lean participants. Thus, favorable changes in measures of cardiovascular health, insulin resistance, inflammation, and obesity were initiated within 8 wk in the OW/OB by replacing deficiencies in Western diets without requiring other dietary or lifestyle modifications; chronic inflammation blunted most improvements.

Major urinary protein 1 interacts with cannabinoid receptor type 1 in fatty acid-induced hepatic insulin resistance in a mouse hepatocyte model

Hepatic insulin resistance (HIR) is a metabolic abnormality characterized by increased gluconeogenesis which usually contributes from an elevation of free fatty acids. Cannabinoid receptor type 1 (CB1R) and major urinary protein 1 (MUP1) are thought to play pivotal roles in mitochondrial dysfunction, liver steatosis and insulin resistance. The aim of this study was to explore the role of MUP1 in CB1R-mediated HIR through the dysregulation of mitochondrial function in AML12 mouse hepatocytes challenged with high concentration of free fatty acids (HFFA). Firstly we observed that treatment of AM251, a selective CB1R antagonist, obviously reversed the HFFA-induced reduction of MUP1 protein expression both in vivo and in vitro. Additionally, our results revealed that AM251 also reverted HFFA-mediated decrease of the mRNA level of mitochondrial biogenesis-related factors, mtDNA amount, ATP production, mitochondrial respiratory complexes-I and -III, and mitochondrial membrane potential, thus consequently might correlate with a parallel reduction of ROS production. Meanwhile, AM251 attenuated HFFA-induced impairment of insulin signaling phosphorylation and elevation of phosphoenolpyrvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase), two key enzymes of gluconeogenesis. Silence of MUP1 gene abolished the inhibitory effect of AM251 on HFFA-mediated elevation of PEPCK and G6Pase expression, whereas the suppression of insulin signaling and mRNA level of mitochondrial biogenesis-related factors were only partially recovered. Altogether, these findings suggest that the anti-HIR effect of AM251 via improvement of mitochondrial functions might occur in a MUP1-dependent manner.

Mitochondria: a possible nexus for the regulation of energy homeostasis by the endocannabinoid system?

The endocannabinoid system (ECS) regulates numerous cellular and physiological processes through the activation of receptors targeted by endogenously produced ligands called endocannabinoids. Importantly, this signaling system is known to play an important role in modulating energy balance and glucose homeostasis. For example, current evidence indicates that the ECS becomes overactive during obesity whereby its central and peripheral stimulation drives metabolic processes that mimic the metabolic syndrome. Herein, we examine the role of the ECS in modulating the function of mitochondria, which play a pivotal role in maintaining cellular and systemic energy homeostasis, in large part due to their ability to tightly coordinate glucose and lipid utilization. Because of this, mitochondrial dysfunction is often associated with peripheral insulin resistance and glucose intolerance as well as the manifestation of excess lipid accumulation in the obese state. This review aims to highlight the different ways through which the ECS may impact upon mitochondrial abundance and/or oxidative capacity and, where possible, relate these findings to obesity-induced perturbations in metabolic function. Furthermore, we explore the potential implications of these findings in terms of the pathogenesis of metabolic disorders and how these may be used to strategically develop therapies targeting the ECS.